usb: dwc3: drd: Defer probe if extcon device is not found

In case the "linux,extcon-name" property is defined but device itself
is not ready, defer the probe.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>